Detailed oxygen and carbon isotope stratigraphy of the Oxfordian in Central Poland

Abstract

A collection of 73 belemnites and brachiopods from detailed stratigraphically documented sections in the Polish Jura Chain of Central Poland representing almost the whole Oxfordian Stage was studied for oxygen and carbon isotopes. The δ18O and δ13C (V-PDB) values range from –3.6 to +0.5‰ and from –1.6 to +2‰ for belemnite rostra, and from –2.5 to +0.4‰ and from +1.6 to +4.3‰ for brachiopod shells, respectively. The δ13C temporal trends of belemnite rostra and brachiopod shells show a constant isotopic difference (ca. 2.5–3‰), which is interpreted as a vital effect of belemnites or as a result of different life environments. δ18O values are similar in both groups of fossils. The δ18O values in the Lower and Middle Oxfordian oscillate around 0‰ and decrease in the Upper Oxfordian. This decrease results mainly from diagenetic overprint as proved by the cathodoluminescence studies. The Lower and Middle Oxfordian δ18O-derived temperatures were constant in consequence of significant ocean depth. For this reason there are no established links with distribution of ammonite faunas. The δ13C temporal trend displays a gentle slope through the Oxfordian and reaches maximum values at the turn of the Plicatilis/Transversarium Zones of the Middle Oxfordian, which is commonly considered as a global event. It can be related to the sea-level highstand and growth crisis of carbonate platforms. The decrease in δ13C values starting in the upper Middle Oxfordian is interpreted to be due to the acceleration of carbonate sedimentation rate during this time period.